Yesterday I was at a local car show and my electrical system was working fine. While I was there somebody wanted to hear my stereo so I played half a song pretty loud with just the key turned backwards. Then when I went to start it later the voltage was bordering in the red on the gauge but the car started right up fine. Then when I went to leave the show the voltage meter still read low and on my way home the dash and gauges all shut off but the car kept running fine. When I got home I shut the car off and checked the voltage at the battery and it was only 9.04 volts. I recharged the battery, started the car everything looked fine but as it idled the voltage kept dropping quickly. I checked the alternator connections and they seemed to be tight, and so were battery cables. Today I charged it to where the battery read 13.41 volts, started it and it instantly drpped to 12.24 volts and stayed there, when I turned my dual Spal fans on it dropped to 12.07 and stayed there and doesnt get any lower. The battery is a red top Optima that is only a year old with 500 miles on it, the alternator is an aftermarket 200 amp model brand new from Excessive Amperage in 2006. What could be going on here is the alternator going bad or is it in the wiring? I don't know what to do here.